This conversion alone can not make it sound "better" ... due to stereo -> mono it can sound DIFFERENT as the soundfield has changed, but no way it can sound better at least when it comes to soundquality. Granted, it can sound better in yours ears depending on audio source (mods have 2+2 channels for example, mixing them as mono might sound more pleasing to ear)
Actually, if he replayed the mp3's on the Amiga with some crap AHI-settings and/or bad mp3-decoder, then the IFF-file might actually sound better on the Amiga
